Cart tests in one file disagree about the same cart

Problem Description

test/cart.test.js is red in CI. Three of its five tests fail, and the numbers in the failure messages are nowhere near the expected ones - one test expects a subtotal of 21600 and gets 75600.

Every one of those tests passes on its own. Running node --test --test-name-pattern='ten percent' is green; running the file is red. Last sprint someone reordered the file to group the discount tests together and a completely different test went red, which is when we started suspecting the tests rather than src/cart.js.

src/cart.js is correct and is shipping in production. The pricing numbers in the tests are the numbers the finance team signed off on, so the expected values are not up for negotiation.

Output Specification

1. Fix test/cart.test.js so all five tests pass in a single node --test run and each also passes on its own.
2. Keep all five tests, their names, and their expected values exactly as they are. Do not modify src/cart.js.
3. Write cart-diagnosis.md: what the tests were operating on, why the failure moved when the file was reordered, and the rule that keeps this from returning when a sixth test is added.

Run node --test before you finish; it must pass.

Input Files

Extract the following files before beginning.

=============== FILE: package.json =============== { "name": "storefront", "version": "3.1.0", "private": true, "scripts": { "test": "node --test" } }

=============== FILE: src/cart.js =============== 'use strict';

function addItem(cart, item) { const line = cart.items.find((i) => i.sku === item.sku); if (line) { line.qty += item.qty; return cart; } cart.items.push({ ...item }); return cart; }

function applyDiscount(cart, percent) { cart.discounts.push(percent); return cart; }

function subtotal(cart) { return cart.items.reduce((sum, line) => sum + line.price * line.qty, 0); }

function total(cart) { const percentOff = cart.discounts.reduce((sum, pct) => sum + pct, 0); return Math.round((subtotal(cart) * (100 - percentOff)) / 100); }

module.exports = { addItem, applyDiscount, subtotal, total };

=============== FILE: test/cart.test.js =============== 'use strict';

const test = require('node:test'); const assert = require('node:assert/strict'); const { addItem, applyDiscount, subtotal, total } = require('../src/cart');

const CART = { id: 'cart-1001', items: [{ sku: 'DESK-1', price: 24000, qty: 1 }], discounts: [], };

test('subtotal covers a single line item', () => { assert.equal(subtotal(CART), 24000); });

test('a new sku is appended as its own line', () => { addItem(CART, { sku: 'CHAIR-9', price: 12000, qty: 1 }); assert.equal(CART.items.length, 2); assert.equal(subtotal(CART), 36000); });

test('a repeated sku merges into the line it is already on', () => { addItem(CART, { sku: 'DESK-1', price: 24000, qty: 2 }); assert.equal(CART.items.length, 1); assert.equal(CART.items[0].qty, 3); });

test('a ten percent discount comes off the subtotal', () => { applyDiscount(CART, 10); assert.equal(total(CART), 21600); });

test('discounts stack additively', () => { applyDiscount(CART, 10); applyDiscount(CART, 5); assert.equal(total(CART), 20400); });